Abstract: Loudness is one of the most essential parameters for assessing the acoustical quality of an auditorium. But owing to the lack of an authentic criterion, it remains unresolved how to evaluate the loudness in concert halls. In this paper L_pF, the mere forte sound pressure level of tutti-sound is suggested as a criterion to describe the loudness in a hall. The prediction procedure of the L_pF value distribution in a hall is demonstrated. Comparison between predicted and measured L_pF levels is given. Tentative optimum and allowable L_pF values for concert halls are also discussed.